1. Field of the invention
The present invention relates to an image forming apparatus which includes a plurality of image forming means and performs image formation using an endless belt member.
2. Description of the related art
A tandem color image forming apparatus, including independent image forming portions with respect to each color of yellow, magenta, cyan, and black, for forming a color image using an intermediate transfer belt of an endless belt member is known as the conventional color image forming apparatus. In image formation, a photosensitive member of each image forming portion is exposed to laser light to form an electrostatic latent image, which electrostatic latent image is then developed with the toner of each color, and the obtained toner image is transferred so as to be sequentially superimposed on the intermediate transfer belt. Subsequently, the superimposed toner images on the intermediate transfer belt are transferred onto a sheet-like recording material all together, and a color image is obtained.
Most of such tandem color image forming apparatus have a configuration in which independent image forming portions are arranged with respect to each color of yellow, magenta, cyan, and black in a line along the rotating direction of the intermediate transfer belt. However, if such image forming portions are arranged in a line, the length of the intermediate transfer belt becomes long, and the color image forming apparatus increase in size along the intermediate transfer belt.
Japanese Patent Application Laid-Open No. 2001-51472 and Japanese Patent Application Laid-Open No. 2002-162807 thus propose to evenly distribute and arrange the image forming portions including the photosensitive member and the like on the opposing outer peripheral surfaces of the intermediate transfer belt to reduce the length of the intermediate transfer belt, and miniaturize the color image forming apparatus.
Higher image quality is being demanded on the tandem color image forming apparatus. Image formation is desirably performed using accessory colors such as light magenta having the same hue as magenta but a weaker concentration, light cyan having the same hue as cyan but a weaker concentration in addition to each basic color of yellow, magenta, cyan and black.
Therefore, when more than four colors are used to form the image, the length of the intermediate transfer belt becomes longer and the color image forming apparatus enlarges along the intermediate transfer belt if the photosensitive members are arranged in a line along the rotating direction of the intermediate transfer belt as in the prior art.
A method of evenly distributing and arranging the image forming portions on the opposing outer peripheral surfaces of the intermediate transfer belt as described in the configuration of the above-mentioned documents is thus considered. In this case, however, the basic colors of yellow, magenta, cyan and black, which are used very often in forming the color image, are distributed and arranged on different surfaces of the transfer belt, and thus the tensile force of the belt at the respective surface tends to differ, which may easily cause color shift. Superimposing the toner images with the basic colors on the same surface of the transfer belt is effective to prevent color shift.
In a configuration in which the basic colors are collected on the same one surface of the transfer belt, however, the spacing between the photosensitive drums, that is, the spacing in the width direction of the image forming apparatus needs to be narrowed, and furthermore, the size of the image forming portion in the width direction needs to be reduced in order to narrow the width of the image forming apparatus. The size in the height direction of the image forming portion must be increased if the size in the width direction of the image forming portion is reduced, and thus the height of the image forming apparatus increases as a result.
Therefore, if the spacing between the photosensitive drums of the accessory colors is made to the same size as the basic color side regardless of the fact that the space in the width direction is greater compared to the basic color side at the surface arranged with the accessory colors fewer than the basic colors, the height of the image forming apparatus increases in order to ensure the optical length of the laser light.